(1) Field of the Invention
The invention relates to gas turbine engine components, and more particularly to systems and methods for determining the location and angular orientation of a hole containing an obstructed opening on a surface of such components.
(2) Description of the Related Art
Many internal components of gas turbine engines are exposed to gas temperatures that exceed their base material's melting temperature. For that reason, internal components such as turbine rotor blades, turbine stator vanes, combustor liners, shroud segments and the like must be thermally protected for improved durability. Typically, internal channels direct lower-temperature air inside these components to reduce their temperature. The lower-temperature air exits the components via a series of small holes, forming a protective film that surrounds the external surfaces of the components. These holes, typically called film-cooling holes, are sized, located and angularly oriented to apportion the lower-temperature air only where it is required. The surfaces are usually further protected with insulating, thermal barrier coatings (TBCs). Typical, state of the art coatings comprise a metallic bond layer and a ceramic top layer. Despite the use of film cooling and thermal barrier coating, some components deteriorate over time and must eventually be restored or replaced altogether. Typically, one or more approved repairs can restore a deteriorated component to a like-new condition at a fraction of the cost of a replacement component.
Conventional restoration of a deteriorated component begins with the removal of the thermal barrier coating by chemical and/or mechanical means. Once the coating is removed, the component is inspected for distress and scrapped if found unserviceable. If the distress is within serviceable limits, the areas of distress and the multitude of film cooling holes are filled using the TURBOFIX® diffusion brazing repair process available under license from the assignee of the present invention. The TURBOFIX® diffusion brazed surfaces are then abrasively blended before new coating is applied. Once the component is coated, each of the film cooling holes is re-drilled using a laser, abrasive water jet, or other suitable drilling means.
In some instances, a component may only require the removal of the coating and application of a new coating to restore the component to like-new condition. Unfortunately, the application of the new coating partially or wholly obstructs the openings of the original film cooling holes. Even the slightest obstruction can negatively affect the film cooling of the component surfaces. In these instances, if it were possible to precisely determine the location and angular orientation of the film cooling holes even though they are partially or wholly obstructed with coating, then the coating could be reamed from the openings using a laser, abrasive water jet or other suitable drilling means. The application of new coating and reaming of the film cooling holes eliminates the time-consuming TURBOFIX® diffusion brazing repair steps. Any reduction in component restoration time or cost significantly benefits a gas turbine engine operator.
There are many challenges involved with determining the location and angular orientation of film cooling holes with obstructed openings. First, the hole diameters are very small, typically less than 0.020 inch. Second, the hole openings are at least partially obstructed with a coating having a thickness of between 0.002-0.020 inch. Third, the hole openings are located on complex, three-dimensional surfaces that may vary slightly from component to component and with extended operation at high temperatures.
One method of determining the locations of film cooling holes uses a manual vision system. According to this method, the locations of the holes are manually located by viewing each hole through a vision system camera that projects a magnified two-dimensional image of the hole opening on a video monitor. This method is labor intensive and since the operator only views a two-dimensional projection from the top of the holes, the hole's angular orientation is not accurately determined.
Another method of inspecting the location of film cooling holes uses an illumination system. According to this method, an array of holes is illuminated from within an internal cavity. An external video camera collects luminance data for display on a monitor and comparison to a reference luminance. This inspection method is useful for determining if a proper size hole is present, but does not verify the exact location and angular orientation of the holes. Also, the method is inoperable for holes that are partially or wholly obstructed with a coating.
Yet another method of inspecting film cooling holes uses an infrared radiometer system. According to this method, hot and cold air is alternately directed into hollow channels within a component and allowed to exit the cooling holes. An imaging infrared radiometer generates a series of images during the heat-up and cool-down cycles. This method is useful for inspecting for the existence of the holes, but does not verify their exact location and angular orientation.
What is therefore needed is an automated system and method for determining the location and angular orientation of holes with openings on surfaces that are at least partially obstructed with a thermal barrier coating.